It is well known that the use of structured packing in distillation columns has many advantages where low pressure drop is important. However, packed column performance is very dependent on creating and maintaining a balance between the downward flow of liquid and upward flow of vapor locally in the packing. The distribution of the liquid and vapor within the packing is influenced by the initial presentation of these fluids to the packing, and the particular characteristics of the packing.
For a variety of reasons, the distribution of liquid within the packing is more sensitive than the distribution of vapor to initial presentation. Typically, initial presentation of liquid is made by a liquid distributor, consisting of a collection of interconnecting open troughs with irrigation holes in the base which feed liquid to the packing below. A uniform liquid level in the troughs of the liquid distributor is a minimum requirement to achieve uniform flow from the irrigation holes.
In practice, difficulties arise in creating uniform initial liquid distribution. For example, a variety of factors affect uniformity of liquid level in a liquid distributor, including hydrodynamic resistance internal to the liquid distributor, misalignment during installation of the distributor, and column tilt. The design of liquid distributors for off-shore applications presents some particularly demanding problems. The action of both wind and waves causes significant movement of a shipboard distillation column; this movement is particularly of concern when it causes the column to tilt away from the vertical in a swaying motion, causing side-to-side movement of liquid within a liquid distributor. The uniformity of liquid level in a typical liquid distributor is significantly compromised during ship movement, causing flow non-uniformity from the irrigation holes, leading to maldistribution within the packed column.
Numerous approaches to providing initial liquid distribution have been proposed including trough distributors, fractal distributors, overflowing distributors, and irrigation-hole-multiplying distributors (see, for example U.S. Pat. Nos. 4,816,191 (Berven & Meyer), U.S. Pat. No. 5,132,055 (Alleaume et al) & U.S. Pat. No. 5,354,460 (Kearney et al), German Published Patent Application No. 2,752,391 (Streuber), and a paper by M. Kearney & V. Kochergin entitled "A Liquid Distributor for Industrial Chromatography Columns: An Approach Based on Fractal Geometry" presented at the Fifth World Congress of Chemical Engineering in San Diago, Calif., Jul. 14-18, 1996). All of these rely to some extent on a uniform liquid level to achieve flow uniformity and are subject to liquid level non-uniformity created by the aforementioned mechanisms. For example, none of these approaches is suitable for a non-vertical column, nor for a periodically tilting column on, for example, a ship.
Alternative approaches to those relying on a uniform liquid head in a trough have been reported. Pluss and Bomio (I. Chem. E. Symp. Ser. 104 (1987) A259) describe a pipe-distributor which was shown to be superior to a trough-type distributor in a tilting column. This pipe distributor is a type of pressurised distributor consisting of a closed manifold, such as a pipe, with irrigation holes directing liquid down into the packing. The number and size of the holes are such that there is a back pressure significantly greater than the difference in head caused by the distributor being tilted. In this way, the distribution becomes relatively even regardless of column tilt.
Unfortunately, there are practical difficulties associated with the pressurised distributor approach. For example, if the distributor back pressure were generated by a pump, the pumped liquid would have to be subcooled to avoid flashing in the pressurised manifold because flashing would lead to maldistribution by formation of bubbles impeding liquid flow in the manifold. Subcooling can be problematical if the liquid is entering the coolest section of the distillation column and there are no cooler fluids to provide subcooling duty. The pumped option is also difficult if liquid is being fed from one section of a column to another, without leaving the column shell.
Another approach, although fundamentally similar to the pressurised distributor, was used by Tanner, et al., (Trans. IChemE 74 (1996) A177) in which a high liquid head in a distributor was shown to yield superior mass transfer performance to a low head in a tilted column, because the variation in head due to tilt on a percentage basis was lower with the high average head than with the low average head. Unfortunately, the elevation required to provide the required back pressures for good distribution is excessive and would severely impact on both column height and cost.
It is an object of the present invention to provide a liquid distributor which permits of uniform distribution to a liquid-vapor contact column during sway or tilt such as that encountered by a shipboard cryogenic air separation distillation column.